A growing awareness of the ecological effects of some toxic metals is evident from various studies and many papers which have appeared during the last ten years on this subject. It has been recognized that biosorbent materials, comprising polysaccharides, proteins and lipids, constitute a potential alternative to conventional methods for the removal of metal ions from effluents and other industrial and domestic waste solutions. Living and non-living cells such as yeasts, fungi, algae and bacteria have been suggested for this purpose, contemplating even a potential method for an industrial recovery of some particular metals, such as mercury, lead and cadmium. The main disadvantage of most microbial cells suggested is the fact that the amount of metal which is adsorbed on the bacteria is relatively small; as a consequence, large amounts of the adsorbent are required. Attempts to increase uptake of metal ions by various treatments have been unsucessful. Furthermore, recovery of the metal ions from the extract by a mineral acid is difficult since the extract also contains other materials extracted from the cells and large amounts of regenerated biosorbent materials will constitute a burden for their disposal. Moreover, these biosorbents can not be recycled. Furthermore, in most cases the microorganisms employed have had to be grown for this purpose, which necessitates considerable investment. In addition, live biomass can not tolerate high concentrations of metals in the effluent.
In a paper by N. Kuyucak et.al. (Biotechnology Letter's 10, 2, 137-142, 1988), Saccharomyces cerevisiae and Rhizopus arrhizus are suggested to be more efficient than algal biomass from Sargassum natans and Ascophyllum nodosum, for sequestering gold and cobalt from solutions. Biosorption isotherms are presented showing the different adsorptions of the various metal ions, using non-living samples of Sacccharomyces cerevisiae.
In a paper presented at a Symposium by P. R. Norris and D. P. Kelly (Dev. Ind. Microbiology 20, 299-308,1979), it is mentioned that the adsorption of metal ions was generally greater and more efficient from solutions with yeasts than with bacteria. Cadmium and copper ions were rapidly accumulated by all of the yeasts, with the exception of relatively poor cadmium uptake by Saccharomyces lipolytica.
Some inhibition of zinc uptake caused by the presence of other cations has also been reported.
In a recent paper by Chu-Pin Huang et al. (Water Research, 24, 4,433-439,1990) the biosorption of copper by virgin or treated unicellular yeast of tile strain Saccharomyces cerevisiae is discussed. The yeast cells were washed, treated with a perchloric acid solution in order to remove surface impurities and further heated at 120.degree. C. to kill the cells before their use in the biosorption process. Huang et al. concluded that for live yeast cells, the uptake of copper takes place in two steps: an initial extracellular adsorption followed by intracellular uptake. However, intracellular uptake should be avoided because a sorbed intracellular metal is difficult to recover by simple acidification.
The above brief review shows the importance attributed to the problem of removing metal ions from aqueous solutions. Considering the severe ecological problems encountered due to pollution by heavy metals, the stringent legislation and implementation, and the measures currently taken to reduce pollution by heavy metals, there is a long felt need for an efficient, cost-effective process for the removal of species containing metallic ions, from effluents. Such a process should use a readily available low-cost biosorbent, and facilitate recovering of adsorbed metals. However, up to now, no such commercial plant exploiting a similar system was known, due to the poor metal adsorption efficiency of the sorbents that had been tested so far, or due to their high production and operation costs.
It is an object of the present invention to provide a simple, fast, efficient and inexpensive process for the removal of species containing metallic ions from effluents by biosorption. It is another object of the present invention to provide a simple process for the removal of species containing metallic ions from effluents, using spent yeasts from the fermentation industry. It is yet a further object of the present invention, to provide a simple process for the removal of species containing metallic ions, where after the sorption of the species containing ions, the contaminated spent yeasts can be recycled for further sorption releasing a high concentration of metallic ion solution, thus alleviating disposal problems and rendering the yeast product suitable for repeated use.